I encountered a performance bug in Pharo 6 under Windows 10 with Moose, starting from the Moose 6.1 (beta) image in Pharo Launcher.

I migrated my code from Pharo 5 and it uses a PharoCommandLineHandler to load an MSE file, which sometimes has many (15K) warnings/errors. It was taking over a minute to run something that in Moose takes only a few seconds.

To debug and make a minimal example, I removed the Moose file part of my CommandLineHandler and simulated the writing of errors (i/o) with a loop. I believe the I/O to the stderr is why the performance is bad. In Windows 10 Task Manager, I can see that the process is not using much CPU. As I recall, my code didn't have this performance problem in Pharo 5.

The command in windows to execute my minimal sample is something like: